Re: [PATCH 09/10] arm64: select DYNAMIC_DEBUG_RELATIVE_POINTERS

From: Nathan Chancellor <natechancellor@gmail.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	Jason Baron <jbaron@akamai.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux@googlegroups.com,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH 09/10] arm64: select DYNAMIC_DEBUG_RELATIVE_POINTERS
Date: Fri, 26 Apr 2019 06:00:15 -0700	[thread overview]
Message-ID: <20190426130015.GA12483@archlinux-i9> (raw)
In-Reply-To: <94b816aa-ad36-75e6-8782-d141dec2e803@rasmusvillemoes.dk>

On Fri, Apr 26, 2019 at 12:05:57PM +0200, Rasmus Villemoes wrote:
> On 26/04/2019 11.39, Arnd Bergmann wrote:
> > On Tue, Apr 9, 2019 at 11:26 PM Rasmus Villemoes
> > <linux@rasmusvillemoes.dk> wrote:
> >>
> >> Similar to GENERIC_BUG_RELATIVE_POINTERS, one can now relativize the
> >> four const char* members of struct _ddebug, thus saving 16 bytes per
> >> instance (one for each pr_debug(), dev_debug() etc. in a
> >> CONFIG_DYNAMIC_DEBUG kernel). The asm-generic implementation seems to
> >> work out-of-the-box, though this is only compile-tested.
> >>
> >> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > 
> > This causes a build regression when compiling with clang,
> > see http://staging-storage.kernelci.org/next-clang/master/next-20190424/arm64/allmodconfig/clang-8/build.log
> 
> Yes, see also https://github.com/ClangBuiltLinux/linux/issues/456 .
> 
> The quickest short-term fix is to append "if CC_IS_GCC" to the select
> statements. Then when a fix lands in clang one can change that to "if
> CC_IS_GCC || CLANG_VERSION >= something". It's probably best if we fix
> -next builds ASAP instead of waiting for knowing the proper value of
> "something". Nathan, Nick, WDYT?

Those select statements have to be added regardless, we might as well do
it now. It should unbreak the auto builders because they use the latest
clang stable version, which is 8.0.0.

For the record, the fix in Clang is https://reviews.llvm.org/D60887 and
should land shortly unless there are any further objections, meaning
this will be fixed in the 9.0.0 release.

'if CC_IS_GCC || CLANG_VERSION >= 90000' should do it on the arm64 and
powerpc select statements (x86 works fine because %c support has always
been present).

> 
> I had 0day verify my patches before sending them out officially, and
> thought it also did clang builds. But apparently not, or not with enough
> arch/.config combinations?

They do not do clang builds unfortunately. Nick was in contact with them
before forced asm-goto on x86 happened and derailed that.

We try to do our own tests and let people know when stuff breaks but
that is usually after it hits -next, rather than the mailing list.

> 
> Rasmus

Cheers,
Nathan